1. Technical Field
The present invention is related to a semiconductor memory device, and more particularly, to a substrate bias voltage generating circuit for use in a semiconductor memory device.
2. Discussion of the Related Art
A semiconductor memory device typically includes a substrate bias voltage generating circuit for generating a substrate bias voltage. The substrate bias voltage is applied to a P-well/substrate surrounding MOS transistors of the semiconductor memory device to obtain some of the following effects.
For example, the substrate bias voltage is applied to a P-well/substrate to increase a threshold voltage of a parasitic MOS transistor. In addition, the substrate bias voltage is applied to a P-well/substrate to reduce the need for increasing the concentration of channel stop implants below a field oxide. This has been shown to improve junction breakdown and reduce leakage current. Further, the substrate bias voltage is applied to a P-well/substrate to reduce increases in a threshold voltage or a body effect of MOS transistors, thus reducing a junction capacitance at an end of a field oxide.
When the substrate bias voltage is applied to a P-well/substrate in a memory cell of a semiconductor memory device, leakage current of a storage node N+ layer connected to a cell capacitor is reduced, thereby increasing data retention time of the memory cell. In addition, since the data retention time is closely related to DRAM refresh time, the applied substrate bias voltage also affects the DRAM refresh time. Further, since the property of a cell transistor having the shortest channel in a chip embodying the semiconductor memory device is improved, a threshold voltage of the cell transistor can be suppressed. This has also been shown to reduce a boosted width of a word line voltage.
Exemplary substrate bias voltage generating circuits are disclosed in U.S. Pat. No. 5,744,997 entitled “SUBSTRATE BIAS VOLTAGE CONTROLLING CIRCUIT IN SEMICONDUCTOR MEMORY DEVICE”, U.S. Pat. No. 6,198,341 entitled “SUBSTRATE BIAS VOLTAGE GENERATING CIRCUIT FOR USE IN A SEMICONDUCTOR DEVICE”, U.S. Pat. No. 6,882,215 entitled “SUBSTRATE BIAS GENERATOR IN SEMICONDUCTOR MEMORY DEVICE”, U.S. Pat. No. 6,906,967 entitled “NEGATIVE DROP VOLTAGE GENERATOR IN SEMICONDUCTOR MEMORY DEVICE AND METHOD OF CONTROLLING NEGATIVE VOLTAGE GENERATION”, and Korea Patent Laid-Open No. 2001-0107692 entitled “SUBSTRATE VOLTAGE SENSING CIRCUIT AND SUBSTRATE VOLTAGE GENERATING CIRCUIT”, the disclosures of which are incorporated herein in their entirety by reference.
FIG. 1 is a block diagram showing a conventional substrate bias voltage generating circuit, and FIGS. 2A and 2B are circuit diagrams showing exemplary embodiments of a detector illustrated in FIG. 1.
As illustrated in FIG. 1, a substrate bias voltage generating circuit 10 consists of a charge pump 12, a detector 14, and a driver 16. The charge pump 12 generates a substrate bias voltage VBB in response to a clock signal CLK, and the substrate bias voltage VBB is supplied to a substrate (not shown). The detector 14 detects whether the substrate bias voltage VBB maintains a predetermined negative voltage, and generates a detection signal DET as a detection result. The driver 16 generates the clock signal CLK in response to the detection signal DET.
The detector 14 may be one of two detector types, they are: an inverter type and a differential amplifier type. An inverter type detector is illustrated in FIG. 2A and a differential amplifier type detector is illustrated in FIG. 2B. Examples of the inverter type and differential amplifier type detectors are disclosed in the above-mentioned references U.S. Pat. No. 5,744,997 and Korea Patent Laid-Open No. 2001-0107692, respectively.
Referring now to FIG. 2A, an inverter type detector 14 receives an internal power supply voltage Vint and a substrate bias voltage VBB to generate a detection signal DET. The detector 14 has a voltage divider structure and generates the detection signal DET as a control signal for operating the charge pump 12 according to the substrate bias voltage VBB.
Since the detector 14 operates simultaneously with the generation of the internal power supply voltage Vint at power-up, a substrate bias voltage of a desired level may be rapidly set up. On the other hand, it is difficult for the detector 14 to stably maintain the substrate bias voltage VBB in view of temperature variations, thus causing deterioration of the DRAM refresh time.
As shown in FIG. 2B, a differential amplifier type detector 14 consists of a voltage dividing section 14a and a differential amplifier section 14b. The voltage dividing section 14a receives a substrate bias voltage VBB and an internal power supply voltage Vint and divides the received voltages VBB and Vint based on a predetermined resistance ratio. The voltage dividing section 14a outputs a divided voltage Vdiv to the differential amplifier section 14b. The differential amplifier section 14b compares the divided voltage Vdiv and a reference voltage Vref and outputs a detection signal DET.
Since the detector 14 in FIG. 2B uses a differential amplifier, it detects the substrate bias voltage more exactly than the detector 14 in FIG. 2A. In addition, since the voltage dividing section 14a generates the divided voltage Vdiv according to the resistance ratio, the detector 14 is capable of maintaining the substrate bias voltage VBB in view of temperature variations. However, the detector 14 in FIG. 2B has a set up time that is slow at power-up.
For example, since the differential amplifier section 14b uses the reference voltage Vref and the divided voltage Vdiv as its input voltages, as illustrated in FIG. 3, the divided voltage Vdiv is lower than the reference voltage Vref until it reaches a predetermined voltage. For this reason, the detector 14 does not operate during a predetermined period of time at power-up, thus resulting in a slow set up time.
Accordingly, a need exists for a substrate bias voltage generating circuit for use with a semiconductor memory device that is capable of maintaining a stable bias voltage during and after power-up.